In recent years, increased is the importance of a strip type immunoassay for immuno chromatography, which does not require pretreatment of a detection sample, as a simple and convenient in vitro diagnostic kit or a portable diagnostic device which detects an antigen in a sample liquid based on utilization of the specific reactivity of an antibody. In particular, a test kit for a pathogen such as a virus or a bacterium is a familiar immunochromatography device widely used in common hospitals or clinics.
The most simple structure of the immunochromatography device is a structure in which a sample addition site, a labeling substance retaining part, a judgment (detection) site, a porous support for immunochromatography, and a sample absorption site are mutually connected. The labeling substance retaining part has been produced by treating with a protective stabilizing solution a labeling substance (hereinafter also referred to as conjugate) labeled with metal nanoparticles for detecting a detection target, subsequently impregnating or coating the porous support for immunochromatography with the substance, and then performing air drying, vacuum drying, freeze drying, or the like.
As the protective stabilizing solution, hitherto, those containing a protein such as bovine serum albumin (BSA) as a protective stabilizing agent have been known but there is a problem that they cannot be maintained stably in a dry state over a long period of time, so that further improvement and investigation have been carried out. For example, those containing casein, a whey protein, or a casein decomposition product have been reported (see Patent Document 1).
Moreover, as protection and stabilization of a labeling antibody in various immunoassays, there has been known a technology of adding a biomolecule (e.g., antibody or the like) to a gold colloid and subsequently protecting and stabilizing the conjugate by incorporating polyethylene glycol substituted with a thiol and/or disulfide group therein (namely, aggregation of particles is minimized and a free surface capable of adsorption is saturated) (see Patent Document 2).
Furthermore, a protective stabilizing solution composed of 20 to 80% of a saccharide such as trehalose and a 0.5 to 2 mol/L buffer solution has been proposed as one capable of stably storing a solid-phase immunoreagent in a dry state for a long period of time (see Patent Document 3).
Moreover, in order to improve stabilization of a sensitized metal colloid-containing freeze-dried product, there is known a technology of incorporating one or more of trehalose, monoarginine glutamate, tryptophan, calcium chloride, and the like therein at the time of freezing and drying a sensitized metal colloid-containing solution (see Patent Document 4).
Furthermore, as an in vitro diagnostic drug composition having excellent storage stability of a protein, especially an enzyme stored in an aqueous solution and showing little decrease in diagnostic accuracy after three months of storage, it has been reported that a problem of inducing aggregation, hydrolysis, and the like of a protein in an aqueous solution can be solved by incorporating guanidine (a salt thereof) or arginine (a salt thereof or a derivative thereof) in addition thereto as a stabilizer (see Patent Document 5).
On the other hand, with regard to an immunochromatography device, it has been a conventionally recognized problem that background coloration (coloration of the part excluding the immobile-phase antibody at the judgment part), blank coloring (coloring of the immobilized phase in the case where a detection substance is absent), and a prozone phenomenon (false negative phenomenon in which a test substance is apparently observed as if it is present in a smaller amount when a large excess of the test substance is used as a sample) not only decrease the SN ratio at detection but also cause malfunction.
The background coloration is caused by hydrophobic bonding between a visualized mobile-phase antibody and a porous carrier, and the blank coloring is caused by an electrical interaction between a mobile-phase antibody having a negative electric charge and an immobile-phase carrier having a positive electric charge, which is also said as nonspecific coloring. Furthermore, the prozone phenomenon is considered to be attributable to a fact that excess test substance that has not been able to react with the labeling substance reacts with the judgment site and the labeling substance labeled with metal particles for detecting the test substance cannot react with the judgment site.
As countermeasures, a number of studies from various standpoints have been performed. For example, in an immunochromatography detection method which makes use of a developing solution containing a pH buffer and the like, various additives have been used for suppression of side reactions arising from biological affinity or for suppression of nonspecific reactions, and examples of such additives include, for promotion of an antigen-antibody reaction or for suppression of nonspecific reactions, proteins (e.g., bovine serum albumin, casein, gelatin, etc.), high molecular compounds (e.g., polyethylene glycol, dextran, methyl cellulose, polyvinylpyrrolidone, etc.), nonionic surfactants (e.g., Tween 20, Triton X-100, etc.), ionic surfactants or polyanions (e.g., dextran sulfate, heparin, polystyrenesulfonic acid, hyaluronic acid, chondroitin sulfate, etc.) or their salts, and the like (see Patent Document 6).
Moreover, there are known an immunochromatography analytic method in which a test substance is developed with a mobile phase containing a basic amino acid (e.g., arginine, lysin, or the like) or an amino sugar (e.g., glucosamine or the like) (see Patent Document 7) and a membrane assay method using an assay medium having an arginine concentration of 0.02 to 1.5M and pH of 7.0 to 9.5 and substantially containing no buffer other than arginine (see Patent Document 8).
Furthermore, as a countermeasure against the background coloration and the blank coloring and for suppressing the side reaction arising from biological affinity, suppressing nonspecific reactions, and stabilizing a conjugate such as the labeling antibody, there may be mentioned proteins (e.g., bovine serum albumin, gelatin, casein, whey proteins, materials containing a casein decomposed product, etc.), high molecular compounds (e.g., polyethylene glycol substituted with a thiol and/or disulfide group, etc.), basic amino acids (e.g., arginine, etc.), and the like.
However, even if the nonspecific reactions are suppressed, there arises a problem that detection sensitivity decreases, and thus the object is not yet satisfactory achieved, so that there is still a similar problem that the nonspecific reactions cannot be sufficiently suppressed.